1. Field of the Invention
The present invention relates to a method of forming a bump for use on a TAB (Tape Automated Bonding) wafer and, more particularly, to a bump forming method compatible with a narrow-pitch inner lead.
2. Description of the Prior Art
A conventional method of forming a bump of this type has been practiced to form a gold bump for gilding an electrode on a wafer or a copper bump for copperplating an electrode on a wafer, but the material cost is disadvantageously high. A ball bump method utilizing a standard gold wire bonder is proposed as a technique for reducing the material cost. FIG. 1 is a plan view showing a state of a ball bump, and FIG. 2 is a side view thereof. A ball bump is formed such that a ball is formed at the tip of a gold wire by a spark rod of a bonder, a gold ball 11 is bonded to an aluminum pad 2 on a wafer 3 under pressure, and the wire is then disconnected. In this method, since a bump formation step can be omitted in a diffusion step, turn-around time (TAT) can be shortened, and wafers having different sizes can be processed, thus providing versatility. The ball bump method has recently received a great deal of attention because the material cost is lower than that of other bump formation methods.
Since a ball 11 is formed at the tip of a gold wire in this conventional ball bump method, the diameter of the gold ball 11 at the time of its formation is about 2.5 times the diameter of the wire. In addition, when the ball 11 is bonded on an aluminum pad 2 under pressure, the diameter D of the collapsed portion of the ball 11 becomes about 3.5 times the diameter d of the wire, as shown in FIGS. 1 and 2. Although the pitch between aluminum electrodes is to be reduced along with the advance of chip micropatterning, the minimum pitch is limited to about 120 .mu.m due to the above reason. To solve this problem, the wirediameter may be reduced. However, a method of forming a ball at the tip of a thin wire requires difficult control of the spark voltage and suffers from poor stability. In addition, the cost performance factor limits the wire diameter of a thin wire to a minimum of 20 .mu.m. In this case, a ball formed at the tip of the thin wire and collapsed on a pad has a diameter of about 70 .mu.m and the limitation of the narrow pitch using the ball bump becomes about 100 .mu.m.